Apparatus for stretch forming hard metal alloys



7 m .W a W, A 2S i Y i O UM A R n @Y T H M LD ,/f AM@ i THQ EGl Nl Bwd .Ee Emu .SF RR o F s U ml. A R .A D.. wm 9 4 @a 1h. L, v 0 N atented Nov. 1, i949 STATES PATENT UFFICE APPARATUS FOR STRETCH FORMING vHARD METAL ALLOYS Application November 24, 1948, :Serial No. 61,802

l This invention relates to sheet metal forming and, more particularly, to an 4improved apparatus for stretching hard metal alloy vsheets into various shapes and forms,

In meeting the present `day demand for streamlined structures, such `as in the forming of covering material for an airplane, it has `been found that the most efficient procedure is to stretch `the metal sheets of which such structures are 4made over `die forms in 'a stretching press.

formed to the desired sha-pe, it is necessary that some 4means be provided for coextensive movement between the sheet metal and the Astretching die or block used. Adhesion between the ,L

surface of the metal sheet and the forming surface causes `an Auneven iiow of the metal worked, producing an unsatisfactory, irregular, wavy surface, finished product. Severe adhesion between the formed sheet and the stretching Vdie Y;

may also cause :ruptures in `the `surface of the sheet or damage to the die `if great care is not exercised in removing the worked sheet. Various methods for attaining free movement ofthe metal during the stretching process have `been developed which work satisfactorily under certain conditions with various results.

One of these prior' methods is to coat the surfaces brought into contact with each other with a lubricant, which method has proved unsatisfactory due to its Yuncf-:rtainty of results and high cost. Another method is that developed by Frank W. Wiesner and Alexander Coulter as described in Patent No. 2,434,379J issued January 13, 1948.

The last aforestated `method `utilizes the flow' characteristics of a rubber sheet placed between the sheet being formed and the stretching block. This method produces satisfactory results in the stretch forming of sheet materials having certain flow characteristics suitable for cold forming, `but is not satisfactory for stretch forming sheet materials that have to be 'worked hot. The working of metal Vsheets formed from hard a1- loys, such as magnesium, has raised a number of problems. One of these problems is to provide a method of forming wherein heat in the sheet can be retained sufficiently long to allow for forming. Another problem is to provide means to prevent adhesion between the ysheet `and the forming means that will withstand the heat involved.

The present invention Jprovides a method whereby a blanket of glass clc-th'or like heat .resistant, hard surface material is .positioned over the stretching block of a stretch press to .act as a heat-insulatingand adhesion-.preventing means between a sheet of heated metal and the forming block with the cloth resisting transmission of heat from the metal `sheet to `the forming block while allowing for movementof the metal vrela- In order to secure an even flow of the metal being 2 'tive to said "block fas the `sheet is being formed.

Itis yamong the "obiects -of our invention to provide an'apparatus for stretching hard sheet metal to various shapes, `including `compound curves, whereby provision is made -to utilize the free flowing characteristics the metal to form to the ldesired sha-pe. The .useof `a sheet or blanket of glass cloth or like material gives the right ydegree of sliding action between the stretching blockand 'the held sheet -to acquire these results and, at the lsame time, removes the possibility of the metal sticking to the forming surface. Such sticking of the sheet met-a1 would result in an uneven thickness of the Ymaterial in the area being formed and a waviness in the surface o'f the 'finished product.

Another object is to -provide an apparatus for stretching hard alloy sheets while hot.

Another object is to provide an apparatus for stretching metal that eliminates the necessity of coating each part with alubricant.

Still another `objec't 'is to provide a method lwhere sheet metal is 'heated immediately prior to being formed and is then `formed over cold forming dies. The use of -a `heat insulating blanket placed 'adjacent the 'forming die eliminates the need for installing costly heating means yin or around the lpress as has been formerly considered necessary in the lforming of certain alloy sheet materials.

A furthercbject lis -to vprovide for the stretching Vof hard metall Asheets that reduces the spoilage and waste -formerly incurred in finishing materials 'in accordance with the `known art. The clean, uniform condition of isheets produced by the method of our invention eliminates waste due to defective surfaces wfhich formerly resulted from the non-uniform lapplication of a 'lubricant 'or the vtoo rapid dissipation of heat.

Further objects, advantages, `and novel features of our invention -will 'be apparent from the follow-ing `description iin which reference is made to the accompanying drawing wherein like num- -bers referto *like parts.

Figure 1 is a perspective `view of the stretching block ycovered bythe stretch blanket of this invention partially -fragmentated `to show its position relative to the bottom of the stretching block.

Figure 2 'is a "partial end View of a stretching press lembodying "our invention with an electrically heated oven positioned `adjacent thereto.

Referring more specifically to `the drawing, we ihave illustrated Ein Figure `1 asheet or blanket of glass cloth lil -or like heat resistant hard surface material having stretch and vfriction characteristics placed overa form -die or stretching 'block Il of a `conventional stretching press with the blanket dimensioned Aso as to exceed the formling surface of the stretching block. The stretchi; ing block Il is supported by a platform I3, as illustrated in Figure 2, arranged to be actuated and forced upwardly by a hydraulic plunger I4. A heated sheet of metal I5 to be formed is secured in the stretching press at opposed edges I2 held by jaws I5 operated by hydraulically actuated clamps I'I supported by a base I8. Sufficient slack is allowed in the central portion of the sheet I5 for shaping it `to the desired form. During the operation of forming sheet I5, the stretching block II with the glass cloth blanket I laid smoothly7 over its forming surface is forced in an upward direction against the inside surface of the sheet by means of the hydraulic plunger I4 acting against the stretching block supporting table I3. The forcing of the table I3 upward presses the stretching block II covered by the glass cloth blanket II] against the formable sheet I5, shaping it to conform to the shape of the stretching block Il. The sheet metal to be formed may be of any strong ductile material, such as steel, aluminum, magnesium, or their alloys. Prior to forming, the sheet I5 is heated sufficiently in a suitable oven to provide the necessary ductility. This may be accomplished by means of any of the different types of heating devices known in the metal working art. However, we have found that a conventional type of oven heated by means of electric strip heaters, or the like, is suitable for use when located near a forming press used for forming hard alloy metal sheets of standard thicknesses. Such an oven I9 is illustrated in Figure 2 as positie-ned on a suitable support means 20 placed adjacent the press.

In stretch forming according to the method of our invention, the procedure is as follows: A quantity of sheets of metal to be formed is inserted in the oven in stacked formation as at 25. The temperature to which the stack 25 is to be heated will depend on the forming characteristics of the metal. For example, in the forming of magnesium alloy sheets of conventional thicknesses, a temperature range of from 550 to 650 F. has been found suitable. Heating the oven I9 is provided for by a top heating unit 2l4 connected to a suitable source of electric energy by wires 22, and the bottom unit 23 is likewise energized through leads 24. The top unit 2l is held in place by suitable side members 2G attached to the inside walls of the oven I9. A sheet I 5 is removed from' the stack 25 in the oven when sufficiently heated for forming and placed in the stretch press with opposed edges I2 gripped by jaws I 6; and the pressing block II covered by blanket II) is forced against the area to be formed causing the metal to flow evenly to the shape of the block which may consist of any number of complex shapes, such as the compound curved form illustrated in Figure 1.

The hard surface of blanket I0 has a low coeflicient of friction that allows for a uniform sliding movement between not only the stretching block and the underside of the blanket, but also the blanket and the sheet being formed. The sliding movement of the metal sheet I5 over the insulating blanket Il) placed adjacent the stretching block facilitates the forming of the metal by preventing any tendency that the metal would otherwise have to adhere to the stretching block. This freedom of movement allowed the metal being formed results in the elimination of any unevenness which might otherwise develop in the finished product. The heat insulating qualities of the blanket I0 are such as to Iact as a heat barrier and to suiciently retard transfer and loss of heat from the metal sheet I5 as to allow for hot forming of the sheet to the desired shape. Upon the completion of the stretching operation, pressure against table I3 is released, allowing the stretching block II' to drop freely away from the formable sheet I5 in its then formed condition. Separation of the forming block II from the formed metal sheet I5 is readily accomplished as the glass cloth blanket I0 prevents any adhesion between these parts leaving the surface of said formed sheet in a smooth, clean condition ready for the next operation without need for degreasing or further conditioning.

Any compression-resistant, hard surfaced, low coefficient of friction material may be used. It has been found that a fabric woven of threads of such material has the necessary exibility to fulll the requirements of element I0. Glass cloth fabric woven from a continuous filament fiber, known in the trade as the electric grade of cloth, has been found to be well suited for the purpose. This type of material not only has an unusually high tensile strength, which resists tearing of the blanket, but also provides an extremely durable hard surface nish which greatly facilitates the desired sliding action between the sheet and forming block so as to prolong the service life of the blanket. The low cost and long life of the material used in the method of our invention as compared with the prior art methods are marked and important features from a large scale production standpoint.

We are aware that prior to our invention, other forming machines have been made with sheet holding devices; therefore, the machine is shown and described for illustrative purposes only. It is also to be appreciated that certain changes, alterations, modications, and substitutions can be made without departing from the spirit and scope of the claim.

We claim as our invention:

A machine for forming a hot metal sheet comprising a movable stretching block having a forming surface, a heat insulating blanket of glass cloth arranged between the forming surface of said stretching block and a sheet of metal to be formed, said heat insulating blanket being unrestrained at the edges and proportioned to exceed the area of the forming surface of the stretching block so as to provide a heat barrier adapted to retard transfer of heat from the hot metal sheet to the stretching block responsive to forming movement of the said block, means fixed with respect to said block to hold said metal sheet by its edges free of the stretching block, and means for moving the block to apply forming pressure against said restrained sheet of metal.

RICHARD E. BERGER. HENRY P. HESSLER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,345,939 Leary Apr. 4, 1844 2,378,413 Lermont June 19, 1945 2,434,379 Wiesner Jan. 13, 1948 2,444,123 Walsh June 23, 1948 OTHER REFERENCES Machinery, July 1944, pp. 177 to 181. Amer. Mach., Nov. 22, 1945, pp. 114 to 117. 

